Fundamentals of Biomedical Engineering

GREY LEVEL THRESHOLDING

1.It is a simplest segmentations process. Many objects or regions are characterised by fixed reflectivity and light absorption by their surfaces. A brightness constant or threshold can be fixed so as to segment or separate the object and its background.

ALGORITHMS

1.Data and an algorithm are two basic related parts of any progrmme. An algorithm is a finite set of instructions if followed can accomplish a particular task. All algorithms must have following criteria :

(a) Input — zero or more. (b) Output — atleast one.

(c) Definiteness — each instruction must be clear and unambiguous.

(d) Finiteness — after a finite number of steps, each instruction must end.

(e) Effectiveness — each operation must be definite and feasible. All instructions must be feasible to be carried out. While creating or using algorithm for digital processing, the principle of human image perception must be followed. As an image is to be understood by a human, the information should be expressed using variables which are easier to perceive like contrast, border, contour and shape texture. The sensitivity of human senses varies logarithmically to the brightness of input signal. Hence after an initial logarithmically transformation, the response to stimuli, may be treated as linear. A few examples of algorithms are :

(a) Algorithm for basic thresholding:

Search all pixel of f (i, j) of image f. An image element g (i, j) of the segmented image is an object pixel if f (i, j) > T and it is a background pixel otherwise.

(b) Algorithm for computing the brightness histogram (i) Assign zero value to all elements of the array hf

(ii) For all pixel f (x, y) of the image f, increase hf (x, y) by 1.

COMPRESSION

1.We have seen that digital image is sampled and mapped as a grid of dots and picture elements (pixels). Each pixel is given a tonal value (white, black, shade of grey or colour ) depending upon brightness which is represented in binary code i.e., zero and one. The binary digits / bits for each pixel are stored in a sequence by a computer. In order to handle large data, data is compressed and reduced by a mathematical representation. The bits are then interpreted and read by computer to produce an analog version for display or printing. The file size of a digital image is very large and it requires very large memory of the computer, thereby taxing the computing and networking capatibilites. Compression is used to reduce the size of image file for storage, processing and transmission. All compressing techniques are based on algorithms which are nothing but mathematical shorthand, abbreviating the long string of binary code of an uncompressed image, thereby creating a compressed image file requiring lesser memory space. Compression can be done by either standard or proprietary techniques. Compression can also be classified as either loss less compression or lossy compression. The loss less compression abbreviates the binary code without discarding any information. Hence on decompression, image is bit for bit identical to the original uncompressed file. The lossy compression uses a method of averaging or discarding the least important on the basis of understanding of visual perception.

Fill in the gaps

1.Two elements for acquiring digital image are sensor and -------. (a) digitizer (b) processor

2.Short term storage device for image processing is computer’s -------. (a) CPU (b) memory

3.The screen of a computer consists of a large number of minute subdivisions which are called -------. (a) pixel (b) bits

4.A frame buffer consists of a large continuous pieces of computer -------. (a) pixel (b) memory

5.The memory bit can be either in zero or

------- state. (a) two (b) one

6.The pixel value in single bit plane can be

-------. (a) one (b) two

7.A four bit planes can have -------

combinations as output at each pixel. (a) 8 (b) 16

8.Incase of colour images, ------- bit planes are required. (a) 4 (b) 3

9.The first step of vision processing is transformation of light energy to array of

-------.(a) figures (b) numbers

10.Data and ------- are two basic related parts of any program. (a) a computer (b) an algorithm

11.------- techniques are based on algorithms

to reduce the image data file. (a) compression (b) shortening

12.Compression can be classified as loss less or ------- compression. (a) gainless (b) lossy

13.The image is processed by a computer by carrying out image digitization, sampling and

-------. (a) quantization (b) manipulation

14.The scanning produces a time varying voltage which is proportation to the image

------- of the scanned spot (a) position (b) intensity

1.Conventional x-ray examination techniques still dominate the field of diagnostic imaging, although most of the images at present being produced are digital. X-rays emanate from a small point source and pass through a portion of the body and onto a detector that records the x-rays that reach the detector as an image which is called radiograph. X- rays radiation is electromagnetic radiation which can ionize the matter through which it passes as it has high energy content. The ionization can cause damage to DNA and cells in human tissues. However it can penetrate the body to allow noninvasive visualization of the internal anatomy of the human body. X-rays also exhibit particle like behavior which are discrete packets of pure energy. These discrete packets are called photons. Inorder to reduce the ill effect of ionization due to x-rays while taking radiography, new x-rays techniques are being developed to minimise the radiation dose. The chief x-rays methods used in the examination are radiography, fluoroscopy, tomography and bronchography.

1.When an electron in an atom transits from outer orbital (higher energy) to inner orbital (lower energy) radiation is emitted. Such a transition of electron can occur, if the atem is in an excited or unstable state and it has a vacancy in the inner electron shell to which the electron can move from the outer electron shell. The emitted radiation can be in the visible, ultraviolet, or x-rays portion of the electromagnetic spectrum. The emitted radiation is called characteristic radiation as its energy content is uniquely characteristic of the atomic species that produced it.

2.There is an another method available to produce x-rays. If an electron beam is accelerated so as to hit a metal target, a shower of radiation is produced by the interaction. If the electron beam is accelerated with enough energy by applying suitable voltage, the radiation produced is x-ray portion of the electromagnetic spectrum.

3.A vacuum tube device as shown in the figure is used to produce x-rays. The tube contains a tungsten filament (the cathode) and a metal

target (the anode) which is also made of tungsten. The filament cathode is heated with electric current. A high voltage is applied between the anode and the cathode. The high voltage facilitates the electrons of the cathode to be drawn off and accelerated towards the anode. The accelerated electrons strike the anode. This results into the production of characteristic x-rays (characteristic of tungsten metal). The x-ray tube is completely enveloped by lead casing on all sides except for a small exit port. The lead casing is used as the lead can absorb most of the emitted x-rays. Hence x-rays can come out of the port only. These x-rays are used for radiography.

X-Ray Tube

OPERATING PRINCIPLE

1.X-rays are absorbed by the body in relation to specific density and atomic number of various tissues. In irradiating a volume of interest, these absorption differences are recorded on an image receptor.

RADIOGRAPHY SYSTEM

1.A high voltage generator as shown in the figure, supplies the essential power to x-ray tube. A collimator is used at the exit port of the x-ray tube to limit the extent of the x- ray field. The x-ray exposure is kept for precise and finite duration by an electronic time switch. The exposure is also autormatically terminated after a certain amount of radiation has been received by the image receptor with the help of phototiming cercuit. The operator selects all operating parameters like exposure and dose of radiation from the operator’s console.

2.An image receptor is a device which can detect and record an x-ray image. It is placed below the patient so that x-ray after passing through patient falls on the image receptor. The patient’s anatomy modulates the intensity of the x-ray field as it passes through his body. The differential x-ray absorption and transmission by tissues of the body results in an exit radiation beam that varies in intensity in two dimensions. The exit radiation beam reaches a detector which detects and records the two-dimensional intensity distribution. The image receptors used in diagnostic radiology can be :

(a) Photographic film, coupled with an intensifying phophor screen.

(b) Storage phosphor screen.

(c) Direct digital readout device.

PHOTOGRAPHY OR X–RAY FILM

1.X-rays normally can not be detected directly by the human sense. Hence indirect methods of visualisation have to be used to see the image of the intensity distribution through the body of a patient. Although x-rays have a much shorter wave length than visible light but x-rays can react with photographic emulsions in same way as it happens incase of visible light. The film exposed of x-ray and carrying an image of the x-rays intensity is processed in the developing solution. The sensitivity of the photographic emulsion can be increased by the use of intensifying screens which are similar to the fluoroscopic screens. The screen is kept into close contact with the film surface so that the film is exposed to x-rays and also to the light emitting from the fluorescent screen. X-ray film is packed in light-tight cassettes with or without intensifying screens. The one side of the cassette is made of thin plastic which can be easily penetrated by the x-rays.

2.An intensifying screen (fluorescent screen) consists of polyster with plastic coating and

a phosphor layer that absorbs x-rays and in response emits visible light. A radiographic cassette consists of a pair of intensifying screens with a sheet of double emulsion film sandwiched between the screens. The film records the visible light image emitted by the intensifying screens in response to irradiation by x-ray. Two phosphor screens are more efficient in detecting x-ray than a single screen. However sharpness decreases.

STORAGE PHOSPHOR SYSTEM (PSP)OR COMPUTED RADIOGRAPHY

1.Storage phosphor or photostimutable phosphor system is used to obtain radiograph in digital form which are suitable for computer based storage and processing. This method is also commonly known as computed radiography. The method also uses a cassette containing a screen coated with phosphor similar to that used in conventional screen film. However, the phosphor used in the intensitying screens emits visible light immediately upon absorption of x-rays which is called fluorescence. The phophor in PSP systems responds to irradiation with x-rays by storing electrical charges in a pattern matching to the pattern of absorbed x-rays intensities. The pattern is read later by a scanning laser device. Laser causes localized heating of the phosphor which leads to stimulation of the metastable trapped charge. The stimulation of the metastable trapped charge leads to conversion of the trapped charge into visible light, which is called delayed luminescence. The visible light is then converted to electric current by a photo multiplier tube which is digitized and stored as a digital image in a computer.This is called computed radiography.

x-rays field

Laser scanning

Unused PSP

Recording the x-rays im age

Laser converts x-rays im age to visible light

1.The direct electronic capture of the radiographic image is the future trend of digital imaging. These will be no need for storage phosphor cassette and subsequent laser readout or digitization of photographic film incase the use of direct digital detector. Concerted efforts are being put up to develop the direct digital detectors. These detectors will convert the radiographic image (the distribution of intensities in two dimensions) into an electrical signal that can be digitized. It is being tried to develop detectors having better spatial resolution and less noise than PSP systems. Such detectors can also be mounted permanently on the x- rays system so as to eliminate cassette handling by any operator.

1.X-rays diverge from a point source, travel in straight lines and these can affect a photographic emulsion. A point source of light produces shadow of a object in its path as shown in the figure. Similarly any object in the path of x-rays casts a shadow which can be recorded on the photographic material as an image. The image formed by x-rays differs from the image formed by the light rays. X-rays can pass through substances which are opaque to light rays. Therefore x-rays can project shadows of structures hidden below the surface of the object and record their images also on to a photographic material.

1.It can be seen that x-rays images are formed by projection i.e., images of objects lying in the path of x-rays are projected on to a photographic material. This differs from the way images are formed on the retina of the eye or on the photographic film in a camera when light rays travel from the object to the recording medium to produce an image which is called a view of the object. The radiographic image produced by x-rays is a projection of the object.

Im a ge

L en s

1.Since the light passes through a lens, the image produced on the retina or on the photographic film is smaller than the object. However in the case of a projected image, a magnified image is formed because the x- rays continue to diverge as they pass from the object to the recording film. The greater is the distance between the object and the film, the greater is the magnification of the image.

x-rays source

FOD

FFD

Object

Im age

Magnification

RADIOGRAPHY

Image Size

Magnification = Object Size

Focus to film distance (FFD) = Focus to object distance (FOD)

IMAGE SHARPNESS

1.The aim of the radiography is to produce an image as sharp as possible. The factors leading to image unsharpness are :

(a) Geometry

(b) Movement

(c) Absorption

(d) Photographic factor

2.Geometric unsharpness. Since x-rays are not originated from a point source but from small area (port of the x-ray tube), this gives rise to geometrical unsharpness to the image as shown in the figure.

Ob ject

Ob ject

Im age

G eom etric unsharpness

Geometric Unsharpness

The amount of geometrical unsharpness increases with increase in source area (focal spot) and increase in object to film distance. Geometrical unsharpness

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MOVEMENT UNSHARPNESS

1.This type of unsharpness results due to patient, equipment or film movement during exposure.

Patient m ovement

M ovement unsharpness

Movement Unsharpness

ABSORPTION UNSHARPNESS

1.If we consider a spherical object of uniform density, absorption will be greatest at the centre and least at the periphery. The gradual fall off in absorption towards the edges leads to the image having an ill - defined boundary which is called absorption unsharpness.

Image Absorption Unsharpness

2.Photographic unsharpness : The intensifying screen contains crystals which fluoresce when irradiated by x-rays. The main reason of photographic unsharpness is the spread of light between the crystals and the photographic emulsion. The spread of light will be greater with larger crystals and increased distance between the crystals.

QUALITY OF DIAGNOSTIC

RADIOGRAPHS

1.High quality of diagnostic radiographs can be achieved by :

(a) Scatter control

(b) Proper radiographic technique

(c) Technical image quality control programme

SCATTER CONTROL

1.In addition to the x-rays that pass straight through the body and contribute to the radiographic image, other scattered x-rays deviating from straight path are absorbed by the image receptor which blur the image, reduce contrast and increase image noise. Scattered x-rays do not contain useful information. Grids are used effectively to control scatter. A grid is a device which is placed directly in front of the image receptor and it consists of a series of closely spaced lead strips. The strips are oriented such a way that x-rays that are scattered from the tissues are absorbed. Only unscattered x- rays can pass through the grid. Hence grid filter blocks out most of the scattered radiation and improves the image quality.

2.Proper Radiographic Technique. Selection of the appropriate parameters of x-rays machine produces an optimally exposed radiograph with acceptable image contrast. The operators should be well trained to know proper radiographic techniques.

3.Technical quality control program. The aim is to optimise image quality while keeping the radiation exposure to patients and staff to minimum. Technical quality control program includes the monitoring of the performance of radiologists and technicians, patient service times, and other performance measures.

FLUOROSCOPY

1.It is a radiological technique by which the deeper structure of the body can be studied under direct vision on a fluorescent screen. The screen consists of a cardboard which is coated with a thin layer of fluorescent material, like zinc cadmium sulphide. The screen is covered with a thin sheet of lead glass through which the light rays can pass. However x- rays cannot pass through the sheet so that fluroscopic image is protected. When the screen is actuated by x-rays, light is emitted reflecting the pattern of the organs of the body through which the x-rays have passed. The fluoroscopic image can be seen more effectively in darkness when the eyes are fully adopted in darkness. The sharpness and contrast of a fluoroscopic image is generally

inferior to those of a good radiogram. The chief advantage is that the fluoroscopy is a real time radiography. Fluoroscopic system allows continuous viewing of a time varying x-ray image and permits live visual evaluation of dynamic events.

2. Modern fluoroscopy systems use a x-ray image intensifier as shown in the figure. The intensifier converts the x-ray energy to visible light. The intensifier is coupled optically to a television camera. The fluoroscopic image is viewed on a cathode

ray tube (CRT) or a video monitor.

TV cam era

G rid

Patient

Operator’s

console

Collim ator

Layout of an Intensified Fluoroscopy System

IMAGE INTENSIFIER

1.The faint image of a fluoroscopic screen can be made brighter with the use of an electronic image intensifier. The intensifier tube contains a fluorescent screen which is coated with a special material to act as photocathode. The electronic image generated on the photo cathode is focussed onto a phosphor screen at the end of the tube by the help of an electrostatic lens system. Due to the acceleration of the electrons by the electrostatic lens system

(–)

Fluorescent screen

with photo cathode x-ray source

Image Intensifier

IMAGE QUALITY

1.The image quality depends upon following :

(a) Contrast. It is the difference in brightness of two neighbouring regions. In grey - scale image where signal differences are represented by varying shades of grey or brightness, high contrast means that two objects of different composition in the image appear very light or very dark. In a lower contrast image, there is less difference in relative brightness.

(b) Noise. It is any signal component in an image that does not convey any useful information. The aim is to have a higher signal to noise ratio to reduce the random noise.

(c) Spatial Resolution. It is ability of an image to faithfully reproduce small details. It is also called sharpness. Unsharpness (blur) indicates lack of spatial resolution.